SADII RESEARCH PROJECT 1 SUMMARY Influenza is an ongoing threat to human health. It challenges the aging immune system every year and causes the highest burden of severe disease and complications in older Americans, particularly those who are older and frail. Annual vaccination remains the main stay of influenza prevention. While efforts to improve vaccine antigenicity are important with certain level of success, it is paramount to address host factors that impact immune responses to the vaccine, as well. We and others have observed vaccine failure in providing acceptable humoral immunity or clinical protection for the frail elderly, indicating the impact of aging and frailty. However, the impact of sex and gender on vaccine-induced humoral immunity and adverse reactions in these vulnerable individuals. Particularly in the context of repeated annual vaccination has not been adequately evaluated. Our ongoing R01-funded 5-year influenza immunization study is targeted to adults over 75 years of age with 125 subjects participated in at least two years (54 males, 43.2%). Preliminary analyses of hemagglutination inhibition titers from the 2015-16 flu season reveal that females were more likely to seroconvert to all vaccine flu strains than males, but these differences were dependent on both age and race. This project builds on the longitudinal component of the ongoing study and proposes an 8-yr longitudinal investigation focusing on!sex differences in vaccine-induced immunity against flu in the frail elderly, with consideration of the intersection of sex and gender with other social stratifiers. It proposes the following aims: 1) to characterize sex differences in antibody responses (NT, HI, ELISA, antibody avidity and IgG subtype) to influenza antigens (HA, NA and M2) and adverse reactions to the annual vaccinations; 2) to characterize sex differences in B cell responses (composition and numbers of memory cells, plasmablasts, and age-associated B cells) specific to HA, NA and M2 to the annual vaccinations; and 3) to explore sex differences in transcriptional activation and B-cell repertoire utilization in response to the annual vaccination. Delineating the impact of sex and gender as important host factors longitudinally in the context of aging, frailty and repeated annual vaccination will shed light on the mechanisms underlying sex-specific vulnerability and facilitate the development of more effective influenza immunization strategy for the most vulnerable older adults.